If you forget to logout of Facebook on a public device or suspect an unauthorized third-party login, the first thing to secure your account is to logout of the social network on all your devices. You don’t need to do this manually. Facebook can identify all devices that are currently logged in and allow you to logout from your account on all of them all at once.
1. Navigate to the Security and login settings page
2. Click "See more" under the "Where you're logged in" section.
3. At the bottom of the list of sessions, click Log Out Of All Sessions.
4. In the dialogue box, click Log Out to confirm the log out action.
That's it, Facebook will simultaneously log you out of all your devices and you can start fresh.